Vacuum gauge



VACUUM GAUGE Filed Sept. 24, 1943 I N VEN TOR.

7w )EARL w. FLOSDORF @294 as. M

ATTORNEY Patented Dec. 26, 1950 VACUUM GAUGE Earl W. Flosdorf, UpperDarby, Pa., assignor to .F. J. Stokes Machine Company, a corporation ofPennsylvania Application September 24, 1943, "Serial No. 503,693

2 .Claims.

This invention relates to vacuum gauges of the "McLeod type.

The underlying principle of a vacuum gauge .of this type is that ameasured volume of highly rarefied gas is compressed to a much smallervolume under the action of definite pressure, and the volume of the gaswhen so compressed is an indication of the pressure of the rarefied gas.Such gauges give accurate readings for noncondensible gases, but thereadings are not accurate where water vapor or other condensible :vaporsor gases are present in the space being evacuated.

An object of my invention is to prevent inaccurate readings of a McLeodgauge .due to the presence of water vapor and other condensible gases.

The reason for the inaccurate reading of the gauge when a condensiblevapor is present is that when the measured quantity of rarefied gas 'orvapor is compressed in the measuring capillary, some of the condensiblevapor or gas condenses and allows the mercury to rise in the measuringcapillary farther than otherwise, and the gauge will thus indicate abetter vacuum than actually exists. Such condensation will take placewhere the condensible gas or vapor is subjected to a pressure exceedingits condensation pressure. Thus, the presence of moisture in the spacebeing evacuated may not only cause inaccurate reading of a vacuum gauge,but it may also cause two gauges on the same system to pro- 'ducedifi'erent readings where the two gauges have different ranges. Forexample, if two gauges are connected to the same vacuum system, onehaving a range from zero to 5000 microns and the other having a range ofzero to 250 microns, the same vacuum indications will not be given onthetwo gauges if water vapor is present. This is due to the difference inpressure exerted bythe mercury column on the compressed vapor in the twogauges. At a temperature of 75 F. the vapor pressure of water is 22 mm.,so that if the vapor is subjected to a pressure of more than 122 mm. thevapor will condense. micron gauge, the reading of 10.0 micronscorresponds to the difference in mercury level of 22 and .on the 250micron gauge reading of "11 microns corresponds to the same differencein mercury level. Accordingly, where there is a substantial amount ofwater vapor present in the gauge and very little air, the vapor willcondense to liquid and the mercury columns in the two capillary tubeswill rise and produce an indication of 100 microns on the wide rangegauge In the 5000 be above 100 microns. The pressure of the air alonewould be below 11 microns, because if it were more it would beimpossible for the mercury to rise in the capillary to that point in the.narrow range gauge.

The error in reading caused by the presence of condensible gases willvary with the temperature, and even where the temperature is known,there is no way of applying a correction factor to the reading tocompensate for the error.

In addition to -water vapor, other vapors will cause inaccuratereadings, including vapors such as alcohol, ether, hydocarbon and otheroils (as from mechanical .or ejector-diffusion types of vacuum pumps),ammonia, amines and vapors of other similar compounds.

In accomplishing the object of my invention I provide a vapor trap inthe connection from the gauge to the space beieng evacuated for thepurpose of preventing the condensible "vapors the pressure to bemaintained in the vacuum space. :My vapor trap includes desiccatingmaterials which effectively remove not only water vapor but othercondensible vapors or gases as well.

Another feature of my invention is to incorporate a vapor trap in aportable McLeod gauge as a component part of a unitary structure.

The accompanying drawing illustrates the preferred embodiment of myinvention.

In the drawing 1'. have shown my invention applied to a McLeod gauge ofthe type shown and described in my United States Patent 2,278,195,although the invention is not limited to this particular construction ofMcLeod gauge. The gauge proper is formed of a glass structure includinga compression chamber or bulb I connected to a levelling bulb orreservoir 2 by tube 2a, the compression bulb being provided with avertical measuring capillary tube la extending upwardly therefrom. Acomparison capillary tube 3 is ar- "ranged in parallel relation with thecapillary 1a and is connected to the lower part of tube 2a. Capillary 3is of the same diameter as capillary Ia so that the capillary depressionefiect will be the same in the two capillaries; otherwise, error will beintroduced in making pressure readings. The upper end of capillary tube3 communicates with a mercury trap chamber 4 which is connected to bulb2 at 4a. A by-pass tube 5 connects the lower end of tube 2a with trapchamber a as shown. This by-pass is of larger internal diameter thancapillaries la and 3. Trap chamber 4 is provided with a nipple ib bywhich the vacuum connection is introduced into the glass structure. Thetube forming nipple 4b is extended inside of the chamber 4 as shown at40 and forms a trap to prevent mercury from escaping from the gaugethrough the vacuum connection. The mercury may be introduced into thegauge and removed from it through a sealedin plug 20 formed in reservoir2.

The glass structure described above is mounted within a casing 6 bysuitable shockproof mounting devices so that the measuring capillary lastands in a vertical position when the casing B is resting on the lowerend as shown in the drawing, and the measuring capillary lies in ahorizontal direction when the casing is turned to rest upon the sideopposite the handle Go. By reference to my United States Patent2,278,195

' it will be understood that the'gauge when mounted in the casing asdescribed is readily portable and that vacuum readings may be obtainedon the gauge by first placing the gauge so that it stands on its side topermit the mercury to 'drain out of the measuring chamber and thenstanding the gauge on its end to;obtain a reading of the vacuum. Also,as pointed out in my prior patent, the operation of the gauge may befacilitated by pivotally mounting the casing 6 on a suitable stand torotate about a horizontal axis. For example, 61) illustrates a part of apivotal mounting secured to the back of the casing 6 for supporting thecasing to be tilted about the axis 60.

The improvement forming the subject matter of the present inventioncompr ses the provision of a vapor trap mounted within the easing 6 andforming a com onent part of the unitary gauge structure. This vapor trapis preferably formed of a clear glass tube I having an opening sealed bya plug 1a and positioned to be accessible throu h an aperture formed inone of the walls of the casing 6, for example the lower end wall asshown in the drawing. The tube 1 is also provided near one end with anip le 1b which is connected by a flexible tubing lb to the ni ple 41)on mercury trap 4. Tube 1 is also provided with a nipple 'Ic adiacentthe rear end. and a flexible tube 8 connects this nipple with the sourceof vacuum to be'measured. Tube 8 may pass out of the casing 6 through ahole formed in the back wall or in one of the end walls or side wallsnot used for su orting the casing. Where the casing is pivotally mountedby means of the pivotal support 619, it is convenient to pass the tube 8out of the casing through an opening formed at the center and passingthrough the pivotal mounting 6b.

is included in the vacuum connection of the gauge, and any gases orvapor which reached the gau e from the space being evacuated must passthrough the trap.

The desiccating material 90 must be effective not only in removing watervapor, but it must iii) also be capable of removing other condensiblevapors. For this purpose, I prefer to use a combination of materials,that is, I use one material which is effective in removing condensiblevapors of all types, and another material which is especially effectivein removing water vapor. I have discovered that desiccating materialswhich are effective for all types of condensible vapors do not havesufficient potential for removing water vapor in sufficient capacity ata properly low vapor pressure to be entirely effective. For this reasonit is necessary to include a second desiccating material which has arelatively large capacity for removing water Vapor. This is especiallytrue since water vapor is the most common vapor encountered in vacuumoperation. In general, it will be found that when non-aqueouscondensible vapors are present there is also present a certain quantityof water Vapor.

For the general desiccant a physical adsorbent material is employed sothat it will have universal action. Suitable materials are silica gel.aluminum oxide, or similar substances having an afiinity for removingall types of condensible vapors. It is also desirable to use a materialwhich is not finely divided and which does not produce dust; for thisreason, silica gel is preferred since it is a non-dusting material.

For the desiccant material specific to water vapor, I prefer to usemagnesium perchlorate, but other materials which may be used are calcium sulphate, barium perchlorate, and phosphorus pentoxide. Thismaterial also should be non-dusting in character. I find that excellentresults are obtained by using magnesium perchlorate in a form which maybe obtained on the market under the trade name of Anhydrone. Thismaterial is effective down to pressures of less than one-tenth of onemicron.

The silica gel alone has a limited capacity for adsorbing water vapor atlow vapor pressures.

For example, silica gel will adsorb water vapor in an amount only afraction of one per cent of its weight before its vapor pressure riseswell above that of a fixed chemical hydrate such as barium perchlorate.Accordingly, the silica gel is not effective in keeping the water vaporpressure at a low value, and the desiccant forming the fixed chemicalhydrate is included in the vapor trap to maintain the water vaporpressure at a low value, that is, at a value not exceeding the vaporpressure of the hydrate being formed by the chem cal combination ofwater with the desiccating material. Magnesium perchlorate has a vaporpres ure which is so low that it cannot be measured by ordinary means atroom temperature, and this material is eiiective to keep the water vaporpressure extremely low so lon as any of the material remains up combinedwith water.

For the pur ose of indicating saturation or exhaustion of thedesiccating materials. a moisture indicator should be included in thevapor trap, and I find that a commercially available indicator materialknown as .Drierite (calcium sulphate with a cobalt salt) is suitable,but any other known moisture indicator may be em ployed which at thepoint of change in color has a vapor pressure hi her than the desiccantspecific to water vapor but lower than the general desiccant. Themoisture indicator may be added as a separate ingredient, or it may beembodied in the magnesium perchlorate 0r calcium sulphate. The latter isa particularly ef-- fective carrier of the indicator so that theroilowing is a preferred combination of chemicals: Anhydrone, indicatingDrierite and silica gel. Preferably equal quantities by volume of theAnhydrone and silica gel are used together with just a few granules ofthe indicat ing Drierite. The chain of action of these substances withwater vapor is that the Anhydrone acts first. This material is utilizedbepending-upon the temperature of the room.

When the calcium sulphate has been exhausted, then the cobalt indicatortakes on water and is quickly exhausted. The fixed hvdrate of thepreferred indicator has a red color which is easily d stinguished fromthe normal blue color of the indicator in i s unhvdrated form. Otherphysical means of distinction may also be utilized, such as di ferencein cr talline form, granular ity, consistency or the like. The order inwhich the various chemicals act to take u the water is the order of thevapor re sures of the hydrates formed. the one. with the lowest vaporpressure acting first. Because of this chain of action, the amount ofcalcium sul hate incorporated in the mixture must be kept at a minimumbecause otherwise with excess capacit in the calcium sulphate, therewould be a fictitious indication of non-exhaust on of mixture at the lowva or pressure of ma nesium perchlorate, and this would defeat thepurpose of the trap.

For satisfactory results under normal operating conditions in the reence of water vapor at re sures on the order of 100 microns up to 2000microns (0.1 to 2.0 mm), a minimum of about two grams of the maior watr-ad orbing substance is required. In the preferred example, this wouldbe two grams of Anh drone. An equal ouantitv by volume of. sil ca el isadvi able. A few granules of indicating Drierite are included. Forpressures lo er than the above ran e. about to /2 of these amounts arerequired.

The above amounts are based on the assumption t at when the gauge is notin u e, undu quantities of water vapor and other conden ibles are notallowed to pass back into the gauge. In other words. it is assumed thatthe gauge is ke t under vacuum practically all of th time. Oth rwise,larger quantities are desirable to avoid freou nt re lacement or reuvenation.

The mixture of desiccating mat rials should be given a preliminarytreatment before being placed in the va or trap, that is, thedesiccating materials should be mixed together, and should be subiectedto a vacuum for a period of two or three hours up to several days. Insuch treatment, the Anhydrone removes all of the moisture from the otherchemicals so that the desiccating mixture is capable of setting up theproper low vapor pressure in the vapor trap.

Among other things, this vacuum treatment of the mixture of chemicalsprovides for the complete dehydration of the silica gel by Anhvdrone andgives the silica gel a much higher effective capacity for othercondensable vapors at proper low vapor pressure than the silica gelotherwise had. Furthermore, in the continued use of the silica gel whenmixed with Anhydrone, the silica gel does not adsorb water vaporabove'the'vapor pressure of Anhydrone so that all of the adsorbingcapacity of the silica gel is, retained for adsorbing other oondensiblevapors;

While the trap has been shown and described as being mounted inside ofthe casing, under certain circumstances it may be mounted outside of thecasing because of the greater accessibility in this position. In anyevent the trap must be connected so that its action is available througha vacuum passageway to the gauge.

From the foregoing it will be seen that th vapor trap will take up alltypes of condensible vapors and will have sufl'iciently strongattraction for water vapor to maintain a properly low water vaporpressure at all times. Since all condensible vapors are excluded fromthe gauge, only dry air or other permanent gases are present in thegauge and errors of conden ation are avoided. Even though thecondensible components of the gas mixture are excluded from the gauge,the readings will be accurate, because the dry air 01' other permanentgas will accumulate on the gauge side of the trap until its pressurebalances the total pre sure of the gas mixture on the other side of thetrap. By the arrangement shown in the drawing, the vapor trap forms acomponent part of a unitary gauge structure and the arrangement is suchthat the trap functions without any attention from the user other thanobservation of possible change in color of the indicator granules fromblue to red. At such time. the chemical most simply is renewed, althoughit may be rejuvenated by suitable treatment, such as heating in vacuo,if desired. The trap does not interfere with the normal operation of thegauge or with the transportation of the gauge from one point to another.

I claim:

I. In combination, a vacuum auge of the Mc Leod type, a vapor trapincluded in the vacuum connection of said gauge for exclud ngcondensible vapors from said gau e and replacing said vapors with dryair, said tra including a material of the adsorptive type having anafiinity for removing all types of condensible va ors and a material forprotecting said adsorptive terial from saturation by water va orcomprising a material forming a fixed hydrate when combined with waterand having a vapor pressure of less than substantially 25 microns atroom temperature.

2. In a vacuum gau e of the McLeod type, the im rove ent which com risesa va or trap included in the vacuum connection of said gauge for excludng condensible vanors from said auge and replacing said vapors with dryair, said trap including vapor-removing materials comprisin magnesiumperchlorate and silica gel.

EARL W. FLOSDOR-F.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Num er Name Date 676,178 Davis June 11, 1901869,925 Moore Nov. 5, 1907 1,170,526 Gaither Feb. 8, 1916 1,231,064Pfanstiehl Oct. 8, 1918 (Other references on following page) 7 UNITEDSTATES PATENTS Number Name Date 1,425,197 Hamlin Aug. 8, 1922 1,535,157Hughes et a1. Apr. 28, 1925 1,798,175 Smith Mar. 31, 1931 5 1,798,733Hasche Mar. 31, 1931 1,950,502 Madan' Mar. 13, 1934 2,147,108 RylskyFeb. 14, 1939 2,210,862 Tronstad Aug. 6, 1940 2,275,719 Bevins Mar. 10,1942 m 2,278,195 Flosdorf et a1 Mar. 31, 1942 2,285,521 Kollsman June 9,1942 Number Number Name Date Stecher et a1 June 29, 1943 McCulloch Nov.20, 1945 Borgstrom Mar. 5, 1946 McCombie et a1 July 6, 1948 FOREIGNPATENTS Country Date Great Britain Mar. 9, 1915 Great Britain 1914 GreatBritain 1931

